If you're looking to learn Nix start-to-end, there manuals for Nix and NixOS, both are good.

This is an extensive study at trying to 'understand' Nix, because is does so many things differently: https://ianthehenry.com/posts/how-to-learn-nix/

If you're just looking to start building your system, follow NixOS manual up until you have a running system, then pace yourself as you will. You can also setup Nix on existing Linux distro or macOS and practice the ropes before plunging into NixOS.

I'd just install nix + home-manager on your linux or Macos box. That's not too hard. Then take one of your dotfiles supported by home-manager e.g. kitty term, and port it to nix + home-manager

No need to boil the ocean, just start small. You can mix and match with what you're using now. If you like, port it all to nix over time